ASTM B499-96
(Test Method)Standard Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals
Standard Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals
SCOPE
1.1 This test method covers the use of magnetic instruments for the nondestructive measurement of the thickness of nonmagnetic coatings over ferrous or other magnetic base metals.
Note 1—Autocatalytically deposited nickel-phosphorus alloys containing more than 8 % phosphorus are sufficiently nonmagnetic to be measured by this test method, as long as the measurement is made prior to any heat treatment.
1.2 These instruments measure either the magnetic attraction between a magnet and the basis metal, as influenced by the presence of the coating, or the reluctance of a magnetic-flux path passing through the coating and the basis metal.
1.3 Measurements made in accordance with this test method will be in compliance with the requirements of ISO International Standard 2178 as printed in 1982.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: B 499 – 96 Endorsed by American
Electroplaters’ Society
Endorsed by National
Association of Metal Finishers
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Measurement of Coating Thicknesses by the Magnetic
Method: Nonmagnetic Coatings on Magnetic Basis Metals
This standard is issued under the fixed designation B 499; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 4. Factors Affecting the Measuring Accuracy
1.1 This test method covers the use of magnetic instruments 4.1 The following factors affect the accuracy of a coating
for the nondestructive measurement of the thickness of non- thickness measurement:
magnetic coatings over ferrous or other magnetic base metals. 4.1.1 Coating Thickness—Inherent to the test method is a
measurement uncertainty that, for thin coatings, is constant and
NOTE 1—Autocatalytically deposited nickel-phosphorus alloys contain-
independent of the coating thickness. The magnitude of this
ing more than 8 % phosphorus are sufficiently nonmagnetic to be
measurement uncertainty is primarily a function of test piece
measured by this test method, as long as the measurement is made prior
to any heat treatment. surface finish (see 4.1.6 on surface roughness). For thicknesses
greater than about 25μ m, this uncertainty is proportional to the
1.2 These instruments measure either the magnetic attrac-
coating thickness.
tion between a magnet and the basis metal, as influenced by the
4.1.2 Magnetic Properties of the Basis Metal—Magnetic
presence of the coating, or the reluctance of a magnetic-flux
thickness measurements are affected by variations in the
path passing through the coating and the basis metal.
magnetic properties of the basis metal. (For practical purposes,
1.3 Measurements made in accordance with this test method
magnetic variations in low-carbon steel can often be consid-
will be in compliance with the requirements of ISO Interna-
ered to be insignificant. To avoid the influences of severe or
tional Standard 2178 as printed in 1982.
localized heat treatments and cold working, the instrument
1.4 This standard does not purport to address all of the
should be calibrated using a calibration standard having a basis
safety concerns, if any, associated with its use. It is the
metal with the same magnetic properties as that of the test
responsibility of the user of this standard to establish appro-
specimen or, preferably and if available, with a sample of the
priate safety and health practices and determine the applica-
part to be tested before application of the coating.)
bility of regulatory limitations prior to use.
4.1.3 Basis Metal Thickness—For each instrument, there is
2. Referenced Documents a critical thickness of the basis metal above which the
measurements will not be affected by an increase in the
2.1 International Standard:
thickness of the basis metal. Since it depends on the instrument
ISO 2178 Non-Magnetic Coatings on Magnetic Substrate—
2 probe (Note 2) and the nature of the basis metal, its value
Measurement of Coating Thickness—Magnetic Method
should be determined experimentally if not supplied by the
3. Significance and Use manufacturer.
3.1 The thickness of a coating is often critical to its
NOTE 2—In this method “instrument probe” will also include the term
performance. For most nonferrous coatings on steel, the “magnet.”
magnetic method is reliable for measuring coating thickness
4.1.4 Edge Effects—The method is sensitive to abrupt
nondestructively and is suitable for specification acceptance
changes in the surface contour of the test specimen. Therefore,
testing and SPC/SQC applications. The test method requires
measurements made too near an edge or inside corner will not
that the magnetic properties of the substrate used during the
be valid unless the instrument is specifically calibrated for such
calibration be the same as that of the test specimen.
a measurement. The effect may extend to about 20 mm from
the discontinuity, depending on the instrument.
This test method is under the jurisdiction of ASTM Committee B-8 on Metallic
4.1.5 Curvature—The measurements are affected by the
and Inorganic Coatingsand is the direct responsibility of Subcommittee B08.10on
curvature of the test specimen. The influence of curvature
General Test Methods.
varies considerably with the make and type of instrument but
Current edition approved April 10, 1996. Published June 1996. Originally
published as B 499 – 69. Last previous edition B 499 – 88. Replaces portions of
always becomes more pronounced as the radius of curvature
A 219.
decreases. Instruments with two-pole probes may also produce
Available from American National Standards Institute, 11 W. 42nd St., 13th
different readings if the poles are aligned in planes parallel or
Floor, New York, NY 10036.
B 499
perpendicular to the axis of a cylindrical surface. A similar either as shims or foils or as coated specimens.
effect can occur with a single-pole probe if the tip is unevenly 5.3.1 Calibration Foils:
worn.
NOTE 3—In the following paragraphs, the use of the word “foil’’ will
4.1.6 Surface Roughness—Measurements are influenced by
imply a nonmagnetic metallic or nonmetallic foil or shim.
the surface topography of the basis metal and coating. Surface
5.3.1.1 Because of the difficulty of ensuring adequate con-
roughness becomes significant when the degree of roughness is
tact, foils are generally not recommended for the calibration of
greater than 10 % of the coating thickness, causing increased
instruments based on the principle of magnetic attraction but
scatter in measurements. Therefore, it is necessary, on a rough
they are suitable in some circumstances provided the necessary
or scratched surface, to make a greater number of measure-
precautions are taken. They can normally be used for the
ments at different positions to obtain an average value that is
calibration of other types of instruments.
representative of the mean coating thickness. If the basis metal
5.3.1.2 Foils are advantageous for calibration on curved
is rough, it may also be necessary to check the zero of the
surfaces and are more readily available than coated standards.
instrument at several positions on a portion of the uncoated,
To prevent measurement errors it is necessary to ensure that
rough, basis metal.
intimate contact is established between foil and substrate.
4.1.7 Direction of Mechanical Working of the Basis Metal—
Resilient foils should be avoided to prevent indentation errors.
Measurements made by an instrument having a two-pole probe
Only nonferrous metal foils should be used for thicknesses less
or an unevenly worn single-pole probe may be influenced by
than 15 μm. Calibration foils are subject to wear and indenta-
the direction in which the magnetic basis metal has been
tion and, therefore, should be replaced frequently. Worn foils
subjected to mechanical working (such as rolling), the reading
shall not be used to calibrate the instrument.
changing with the orientation of the probe on the surface.
5.3.2 Coated Standards—These calibration standards con-
4.1.8 Residual Magnetism—Residual magnetism in the ba-
sist of coatings of known, uniform thickness permanently
sis metal affects the measurements made by instruments which
bonded to the substrate material.
employ a stationary magnetic field. Its influence on measure-
5.4 The basis metal of the calibration standards shall have
ments made by reluctance instruments employing an alternat-
magnetic properties similar to those of the basis metal of the
ing magnetic field is much smaller.
coated test specimen. To confirm their suitability, a comparison
4.1.9 Stray Magnetic Fields—Strong stray magnetic fields,
of the readings obtained with the basis metal of the bare
such as are produced by various types of electrical equipment,
standard and that of the test specimen is recommended.
can seriously interfere with the operation of magnetic thickness
5.5 In some cases the calibration of the instrument should be
instruments.
checked by rotating the probe in increments of 90° (see 4.1.7
4.1.10 Foreign Particles—Magnetic instruments of all
and 4.1.8).
types must make physical contact with the test surface and are,
5.6 The basis-metal thickness for the test and the calibration
therefore, sensitive to foreign material that prevents intimate
shall be the same if the critical thickness, defined in 4.1.3, is
contact between probe and coating surface. Both the test
not exceeded. It is often possible to back up the basis metal of
surface and instrument probe should be kept free of foreign
the standard or of the test specimen with a sufficient thickness
material.
of similar material to make the readings independent of the
4.1.11 Conductivity of Coating—Some magnetic instru-
basis-metal thickness.
ments work at frequencies between 200 and 2000 Hz. At these
5.7 If the curvature of the coating to be measured is such as
frequencies, e
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.